The present invention is directed generally to a method and apparatus for removing obstructions in mines and specifically to a system for removing rock blockages and/or oversized and/or unstable rock masses in mines and other types of excavations.
In mining applications, it is common to encounter rock blockages of mine openings, such as shafts, adits, stops, drawpoints, and drifts, and oversized and/or unstable rock masses such as in large surface mining and quarrying operations. Such rock masses can interrupt production and pose an unsafe condition for employees.
The removal of such rock masses is not only extremely hazardous but also difficult. Typically, personnel must approach and inspect the rock mass, sometimes drill one or more holes into the rock mass, and implant explosives that will cause removal of the rock mass. People have been killed or seriously injured while performing these steps.
In designing a system for removing such rock masses, there are a number of considerations. First, the system should be capable of remote operation to reduce the hazards to personnel. In other words, the system should be capable of being controlled remotely (e.g., positioned, aimed, and/or fired remotely from the location of the system). Second, the system should be relatively inexpensive in the event that the rock mass, when released, buries the system. Third, the system must have a low rate of misfires. Fourth, the projectile fired from the system should disintegrate upon impact in the event that a misfire occurs and thereby dissipate the explosive charge and render harmless the undetonated explosive charge. Fifth, the system should be relatively accurate in striking the rock mass with the projectile over a substantial distance. Finally, the system should provide for ease of use, be of robust construction, and be simple in design and cost effective.
The present invention provides a system for launching a projectile to explode on impact and break rock in mines and other excavations. In one embodiment, the system includes:
(a) a projectile having:
(i) a nose that is substantially flat or concave to inhibit deflection of the projectile from a face of the rock;
(ii) a body containing an explosive charge; and
(iii) a tail having a plurality of transversely oriented fins to control the trajectory of the projectile; and
(b) a tube for launching the projectile. The system is simple and safe to use, cost-effective, of robust construction and highly effective and efficient in removing obstructions and enables accurate and remote shooting of rock masses, even of high rock hangups.
The body of the projectile contains a detonating device having a detonator inserted into its front end, a striker in its rear end, and a primer located between the detonator and striker. The striker and primer are separated from one another by a spring member which forces the striker away from the primer and a safety pin which restricts the motion of the striker towards the primer during shipping. The safety pin is removed before the launch of the projectile to permit the striker to impact the primer upon impact of the projectile with the rock face. Upon impact with the rock, the striker is forced forward with a sufficient force to overcome the resistive force of the spring and impact and ignite the primer which in turn ignites the detonator. The safety pin can be highly effective in preventing misfires of the detonating device during projectile assembly.
The relationship between the mass of the striker and the spring constant is an important consideration. Preferably the mass of the striker ranges from about 0.5 to about 7 grams and the spring constant from about 15 to about 30 lbs/inch.
The body of the projectile also contains an explosive charge, preferably castable, that is in contact with the detonating device. The explosive charge can be any suitable explosive and preferably is selected from the group consisting of TNT, PETN, RDX, HMX, ammonium nitrate-based explosives, and mixtures thereof.
The explosive charge and detonating device (which includes the detonator) are located in the forward section of the body to permit the charge and detonating device to be disintegrated upon contact with the rock mass. The walls of the body are preferably formed of plastic or another brittle material and have a thickness ranging from about 1 to about 6 mm to facilitate the disintegration of the projectile in the event of a misfire.
Typically, the detonator is inserted into the body of the detonating device immediately before the detonating device is inserted into the projectile. The detonating device (minus the detonator), the detonator, the projectile body and pusher plate, and the explosive charge are shipped separately and assembled at the site. This is done by placing the detonator in the detonating device; placing the detonating device into a passageway in the projectile body for holding the detonating device, and placing the explosive charge in the front of the projectile to form the fully assembled projectile.
The detonating device is received in a pocket in the body that permits the detonating device to move longitudinally and latitudinally in response to movement of the projectile. In this manner, the possibility of a misfire is significantly reduced, even at low flight velocities. The movement of the detonating device within the pocket will permit the striker to more readily impact the primer.
The body also can include a plurality of ribs to support the explosive charge upon impact with the rock mass. Preferably, 6 or more ribs are used to inhibit the explosive charge from deforming and flowing into the gaps between the ribs.
The center of gravity of the projectile is preferably located in the body section and the center of pressure preferably in the tail section to provide more desirable flight characteristics. Thus, the center of gravity and center of pressure are longitudinally of offset from one another along the longitudinal axis of the projectile. To accomplish this result, the outer diameter of the projectile body is no less than about 25% and no more than about 100% of the outer diameter of the tail section and the length of the projectile body is no more than about 50% of the length of the tail.
The launching tube includes a cavity at a bottom of the tube for containing a propelling charge for launching the projectile from the tube. The propelling charge is a suitable energetic substance such as a propellant or an explosive.
A pusher plate is located between the propelling charge and the bottom of the projectile. The pusher plate detachably contacts the bottom of the projectile. The pusher plate is a solid disk that substantially fills and substantially seals the portion of the tube below the pusher plate. As a result, a pressure differential exists across the pusher plate upon ignition of the propelling charge, with the pressure in the cavity beneath the pusher plate exceeding the pressure in the tube above the pusher plate. The pressure differential pushes the pusher plate and projectile from the tube at a velocity in excess of about 25 m/sec.
The firing tube and/or projectile can include remote control components to permit remote firing, arming, and detonation of the projectile. By way of example, the tube can include a receiver/transmitter for receiving a control signal from a transmitter held by an operator and transmitting a second control signal to a receiver in the projectile and/or to initiate the propelling charge and thereby fire the projectile. The projectile can include at least one receiver unit for receiving the control signal from the transmitter in the tube or the transmitter held by the operator. The receiver unit can in turn generate a control signal to pre-arm, arm, or initiate the detonating device. The projectile can also include one or more counters to determine a time interval after the firing of the projectile from the tube and provide a control signal to fully arm the detonating device or detonate the detonating device after a predetermined time interval has elapsed.
In another embodiment, the present invention provides a method for removing a body of rock in an excavation. The method includes the steps of:
(a) aiming a firing tube containing a projectile such that the projectile impacts a preselected target area on the rock body after launching;
(b) transmitting a control signal to a receiver from a remote location to cause at least one of the following to occur: firing of the projectile and arming of the projectile;
(c) firing the projectile from the tube; and
(d) contacting the nose of the projectile with the target area.
Typically, the velocity of the projectile after leaving the tube is no more than about 250 m/sec and more typically ranges from about 25 to about 150 m/sec.
Aiming of the device underground or at night is relatively straightforward. A radiation emitting device, such as a flashlight or laser, is detachably mounted onto the tube and a light beam from the device is aligned with the desired target area to align the launching tube with the target. This methodology is highly accurate and reduces the likelihood that the projectile will miss the target area.
The method can further include steps to arm and detonate the projectile remotely. By way of example, the method can include the steps of transmitting a second control signal when the projectile is fired to a counter and when the counter determines that a predetermined time interval has elapsed, generating a third control signal to perform at least one of the following steps: closing a final arming switch for a detonating device in the projectile and initiating the detonating device to ignite an explosive charge in the projectile. The method can include the steps of converting the control signal into electrical energy and, when a predetermined amount of electrical energy is generated in the converting step, transmitting the electrical energy to a firing device to initiate the firing step or to an ignition device in the projectile.